Exhaust gases of Diesel engines contain PM, which may cause environmental problems. To trap the PM, a Diesel particulate filter (DPF) was designed, in which the PM is filtered from the exhaust gas. The most common type of DPF is a ceramic wall flow filter made of SiC or cordierite. Because the collected PM is accumulated in this filter, the backpressure is increasing and the power of the engine is decreasing. Therefore the wall flow filter is to be regenerated continuously (passive) and/or discontinuously (active) to burn off the soot (see e.g. A. P. Walker et al, Controlling particulate emissions from Diesel vehicles, Topics in Catalysis Vol. 28, 2004, 165-170).
For example, in U.S. Pat. No. 8,114,354 it is described that for the passive regeneration the filter has to be washcoated with a catalytic composition. This catalytic composition, comprising elements of the group of Al, Ce, Zr, Si, zeolites, base metals and noble metals, oxidizes the NO to NO2, which is a better oxidant for soot than oxygen. The active uncatalyzed regeneration operates around 650° C. Postinjection of fuel directly in front of the Diesel Oxidation Catalyst (DOC) is the most common method to reach this temperature. The postinjection method has the disadvantages of an increased consumption of fuel and of the dilution of the motor oil.
According to the presentation given by S. Spiess (Umicore AG) at the CAPoC9 (August 2012) gasoline direct injection vehicles are gaining market share to reach new CO2 emission limits. Through this new technique the fuel consumption and CO2 emission can be reduced, but the vehicles release about significantly more particles than traditional gasoline engines and 10 times more than new Diesel engines. This pollution can be prevented with a gasoline particulate filter (GPF) which can significantly reduce the emissions of particles.
Efforts have already been made to provide a catalyst for the catalytic oxidation of PM with oxygen at lower temperatures to decrease the amount of active regeneration cycles and regeneration time. Thus, the fuel consumption could be decreased and the operation performance of the motor oil can be increased.
In DE 102 00 900 2182 various noble metal free mixed oxides are disclosed for soot oxidation. Oxides on base of iron, chromium and cobalt are disclosed.
EP 2 210 861 relates to a Diesel particulate filter comprising a cerium-containing composite oxide with Bi and Pr, wherein the molar ratio of Ce, Bi, Pr is expressed by Ce:Bi:Pr=(1-x-y):x:y. wherein 0<x≤0.3 and 0<y≤0.5.
In U.S. Pat. No. 8,071,501 an exhaust gas purification catalyst comprising a composite oxide and a Platinum Group Metal (PGM, wherein PGM includes Ru, Rh, Pd, Os, Ir and Pt) is described, wherein the composite oxide consists of Ce, Bi and a lanthanide except La and Ce. In EP 2 269 730 these kind of composite oxides were extended by a further element selected from the group 3, 4 and 13 of the periodic table of elements.
In US 2009/0288401 there is disclosed a composite oxide for exhaust gas purifying catalyst containing Ce, Bi and one or more elements selected from the earth alkaline metals except beryllium. Additionally one or more further elements can be selected from Zr, Pr and Tb.
Perovskite type composite oxides for soot oxidation are claimed in EP 1 829 609. These perovskite compositions may be represented by the structural formula RTO3, where R comprises one or more elements selected from a group made up La, Sr, Ba, Ca and Li; and T comprises one or more elements selected from a group made up of Mn, Fe, Co, Cu, Zn, Ga, Zr, Mo, Mg, Al and Si.
In WO 2006/044822 (EP 1 817 096) there is described a catalyst for the oxidation of soot consisting of an alkaline metal, cerium, oxygen and optionally a platinum group metal and/or zirconium. The most active materials are described to be combinations of cerium and potassium or cerium and caesium.
In Kripasindhu Sardar et al, Nanocrystalline Cerium-Bismuth Oxides: Synthesis, Structural Characterization, and Redox Properties”, Chemistry of Materials, vol. 22, no. 22, 23 November 2010 (Nov. 23, 2010), pages 6191-6201, ISSN: 0897-4756, doi: 10.1021/cm1025848 there is described a Cerium-Bismuth mixed oxide with a background level of sodium. of sodium.
In EP 2 098289 a composite oxide for use in an exhaust gas purifying catalyst, containing Ce, Bi and one or more elements selected from Mg, Ca, Sr and Ba is disclosed. That catalyst is disclosed to be suitable to burn off PM of diesel exhaust gas at low temperatures and to be hardly poisoned due to sulfur oxide action.